Metallic sheet covered with polyester resin film and having high workability, and method of manufacturing same

ABSTRACT

The present invention relates to a polyester resin-covered metal sheet exhibiting superior formability and particularly suitable to withstand the stresses of drawing, drawing and ironing, drawing and stretch forming, or drawing and stretch forming followed by ironing. The resin-covered metal sheet is produced by contacting a biaxially oriented polyester resin film to a metal sheet heated above the melting temperature of the resin, covering the metal sheet with the polyester resin, pinching and press laminating the film-metal sheet composite, and cooling the laminate at a rate of 600° C./second immediately thereafter. It is essential that the resulting resin-covered metal sheets exhibit a true stress of 3.0 to 15.0 kg/mm 2 , corresponding to a true strain of 1.0 at 75° C. These laminates also exhibits a gradation of retention of the polyester resin&#39;s original biaxial orientation. Portions of the resin farthest from the metal sheet retain the greatest degree of their original orientation, the resin structure becoming increasingly amorphous nearer the underlying metal sheet.

FIELD OF THE INVENTION

The present invention relates to a polyester resin-covered metal sheetpossessing excellent formability. The covered sheet is particularlysuited for “heavily formed” use, such as drawing, drawing and ironing,drawing and stretch forming, and ironing after drawing and stretchforming. The present invention also teaches a method for forming saidresin-covered metal sheet.

BACKGROUND OF THE INVENTION

Metal containers such as beverage cans or battery containers aretypically formed by drawing, drawing and ironing, drawing and stretchforming, or ironing after drawing and stretch forming. These drawingprocesses expand the interior volume of the metal container by reducingthe thickness of the surrounding walls. Subsequent to drawing, thecontainers are usually laminated with a corrosion resistant coating andprinted with desired text and indicia.

This process may be enhanced by first coating the metal sheets with anorganic resin, such as polyethylene terephthalate (PET). Initial coatingof the subsequently-drawn metal reduces coating costs and mitigatesenvironmental pollution resulting from dispersion of solvents during theapplication of corrosion resistant coatings. Such resin-coated metalcans have already been utilized in beverage cans.

Suitable organic resins, in the form of a biaxially-oriented film, areheat-bonded to metal sheets prior to the drawing process. These filmsare manufactured via biaxial elongation of a thermoplastic polyesterresin, followed by heat-setting. Their mechanical characteristics, whenmeasured with a tensile tester, are characterized by large yieldstrength and small elongation (elongation after fracture).

Alternatively, the resin films may be laminated onto a metal sheet withadhesive, so as to avoid the loss of biaxial orientation that resultsfrom heat-bonding. However, due to their limited ability to elongate,these resin coatings exhibit numerous fractures and cracks. Furthermore,where there is only limited adhesion between the metal and the resin,the resin tends to peel off during the lamination process. Furthermore,where heat bonding is utilized to laminate the polyester resin onto themetal sheet, the biaxial orientation of the resin film is partially orentirely lost. Consequently, the yield strength of the post-laminationresin film decreases while elongation improves, preventing the film fromcracking, peeling-off, or fracturing. Conversely, resin films lackingbiaxial orientation have such large permeability that the contents of acontainer laminated therewith permeate the film and corrode the metalsubstratum. Such films also tend to generate coarse spherlites duringthe printing process, and tend to crack readily if containers collide orfall.

In biaxially oriented polyester resin films heat bonded to metal sheets,the elongation after fracture, prior to lamination, is defined in one ofthe following ways:

1. According to the preferable range described in Laid open Japanesepatent Hei 1-249331,

2. As the range of the orientation coefficient showing the degree ofbiaxial orientation prior to lamination,

3. As the preferable range of elongation after the fracture and tensilestrength are defined, as illustrated in Laid open Japanese patent Hei2-70430.

Processes like those disclosed in Laid open Japanese patents Hei1-249331 and Hei 2-70430, which utilize heat bonding to laminate abiaxially oriented resin film onto a metal sheet, effectively destroythe resin's biaxial orientation. This alters the values of thepost-fracture elongation and the tensile strength. Thus, previouslyacceptable, biaxially oriented films, subsequent to heat lamination, mayno longer exhibit the same favorable biaxial orientation; the film'sfavorable elongation and tensile strength will also be compromised.

OBJECT OF THE INVENTION

It is an object of the present invention to overcome the deficiencies inthe prior art. Accordingly, the present invention produces a polyesterresin film-covered metal sheet

The values of the elongation after fracture (disclosed in Laid openJapanese patent Hei 1-249331) and elongation after fracture anddisclosure of tensile strength (Laid open Japanese patent Hei 2-70430)are determined prior to lamination of the resin to the metal sheet.characterized by excellent formability and adapted for use inconventional drawing, drawing and ironing, drawing and stretch forming,and ironing after drawing and stretch forming processes.

SUMMARY OF THE INVENTION

According to the present invention, a polyester resin film-covered metalsheet, which retains the biaxial orientation of the resin subsequent tolamination, has a true stress value ranging from 3.0 to 15.0 kg/mm²measured at 75° C. and corresponding to a true strain of 1.0. In apreferred embodiment, the polyester resin is a polyethyleneterephthalate resin having a low crystallization temperature, (i.e., thetemperature of the exothermic peak produced upon heating a quenchedsample of the resin in a differential scanning calorimeter) between 130and 165° C., preferably 140 to 155° C. The polyester resin is preferablya copolyester resin of recurring ethylene terephthalate or butyleneterephthalate monomers. Alternatively, it may be a copolyester resinconsisting of at least two of the ethylene or butylene terephthalatemonomers, or a double layered polyester resin consisting of a laminateof at least two of the afore-mentioned resins.

The method of producing the present invention entails contacting thepolyester resin to a metal sheet, heating the composite to a temperatureabove the melting temperature of the polyester resin, and pinching andpressing the composite into a laminate with a pair of laminating rolls.The laminating rolls form a nip at the exit site of the laminate, saidnip being equipped to cool the emerging laminate at a rate of at least600° C./second. The resulting laminate exhibits a true stress of 3.0 to15.0 kg/mm² measured at 75° C. and corresponding to a true strain of1.0.

The resin contemplated for use in the present invention consists of apolyester resin, preferably polyethylene terephthalate, having a lowcrystallization temperature ranging from 130 to 165° C., optimallybetween 140 to 155° C. Alternatively, the resin may constitute acopolymer of ethylene terepthalate and ethylene isophthalate monomers.Either formulation, when applied to a metal sheet according to thepresent invention, results in a laminate of decreased yield strength andincreased elongation, thereby reducing occurrences of film peeling,cracking, or fracture.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the coating of metal sheets with apolyester resin film, preferably polyethylene terephthalate (PET),biaxially oriented along its length and width, and having a lowcrystallization temperature ranging from 130 to 165° C., optimallybetween 140 and 155° C. This latter value will be explainedsubsequently.

When an amorphous polyester resin, such as PET, is obtained by heatingsaid resin above its melting temperature, immediately quenching theresin, and then gradually heating with a differential scanningcalorimeter (DSC). This process generates an exothermic peak betweentemperatures of 100 and 200° C., depending upon the resin composition.Resins with exothermic peaks at higher temperatures exhibit lowercrystallization velocities compared with those characterized bylower-temperature exothermic peaks. For example, polybutyleneterephthalate resins produce an exothermic peak at about 50° C., whereasPET generates a peak at about 128° C. In contrast, an ethyleneterephthalate -ethylene isophthalate copolyester resin (typically usedin “2-part” cans) exhibits an exothermic peak at about 177° C.

According to the present invention, a biaxially oriented film of PETresin having a low crystallization temperature outside the 130 to 165°C. range can be heat bonded to a metal sheet. However, a PET resinexhibiting a crystallization temperature between 130 and 165° C. isbetter suited to produce a metal-resin laminate that retains the biaxialorientation of the resin, as well as impermability and impactresistance.

Any resin films of PET, polybutylene terephthalate, ethyleneterephthalate copolyesters, ethylene isophthalate copolyesters, blendedpolyesters of two or more of the preceding components, or multilayers ofthese resins are applicable to the present invention. Where superiorimpact resistance is required, a bis-phenol A polycarbonate resin may beadded to the polyester resin. Alternatively, the bis-phenol Apolycarbonate resin or the bis-phenol A polycarbonate resin incombination with the aforementioned resins of the present invention, maybe incorporated into the center of into a multilayer resin, theoutermost layers consisting of the aforementioned resins of the presentinvention. Colored resins may be produced by adding pigments to themolten resin during its manufacture.

The thickness of the polyester resin film ranges between 5 and 50 μm,preferably 10 to 30 μm. Thinner films tend to wrinkle and fail toprovide uniform coverage of the metal sheet. Films exceeding thicknessesof 50 μm are unnecessary and economically inefficient.

It is essential to the present invention that the resin employed thereinhas a true stress of 3.0 to 15.0 kg/mm² at 75° C., corresponding to atrue strain of 1.0. Metal sheets covered with such polyesters withstand“severe forming” methods such as drawing, drawing and ironing, drawingand stretch forming, and ironing after drawing and stretch forming.These drawing methods are carried out at temperatures exceeding theglass transition temperature of the polyester resin, enhancing theformability thereof. The precise technique for heat bonding a polyesterresin film to a metal sheet will be described subsequently.

The true strain and true stress of the disclosed polyester resins aremeasured according to the following procedure:

A resin covered metal sheet is immersed in hydrochloric acid solution,dissolving the metal sheet so that only the polyester resin filmremains. A test piece of this film, measuring approximately 5 mm inwidth and 50-60 mm in length, is subjected to a tensile tester at atemperature of 75° C. A cross head distance of 20 mm is maintained. Fromthese parameters, the nominal stress, σ₀, and the elongation of theresin, E1, are calculated according to the following formula:

E1=100×(L−L0)

wherein

L0: the length of a test piece before stressing

L: the length of a test piece after stressing true strain, εa and truestress σa, are calculated as follows:

εa=ε/(1+ε)

σa=σ ₀/(1+ε)

wherein

ε: strain

ε: E1/100.

The value of true stress, corresponding to the true strain of 1.0, maybe obtained from the graph of the true strain-true stress curve.

The resin of the present invention preferably exhibits a true stressvalue between 3.0 and 15.0 kg/mm². Resins with lesser true stress valuesproduce uneven coverage of metal sheets, due to the large coefficient offriction that develops between the resin and the laminating machinery.Additionally, these resins lack the impermeability to insulate the metalfrom corrosion. Resins characterized by a true stress value in excess of15.0 kg/mm² tend to crack extensively during lamination, again resultingin uneven coverage of the metal sheet.

In cases where there the resin does not sufficiently adhere to the metalsheet, or where single-ply lamination fails to provide adequatecorrosion protection and impact resistance, a thermosetting adhesive(e.g., phenol-epoxy adhesive) is coated on a surface of the metal sheetand dried. Alternatively, the polyester resin film, prior to lamination,may be coated with the thermosetting adhesive. Application of theadhesive, however, often proves expensive, and the solvents usedtherefor are often detrimental to the environment. It is preferred,then, that the additional step of coating the resin or metal withadhesive be avoided whenever possible.

The metal sheets contemplated by the present invention include surfacetreated strips or sheets of steel or aluminum alloy. If steel is used,low carbon, tin-free steel, having a thickness between 0.15 and 0.30 mm,is preferred. A two layered coating of metallic chromium (bottom layer)and hydrated chromium oxide (upper layer) is applied to the steel sheetto facilitate subsequent adhesion of the polyester resin. Thechromium-chromium oxide coating may also be applied to steel sheetsplated with tin, nickel or aluminum, a double layered plating or alloyplating of more than one of tin, nickel or aluminum.

If aluminum alloy is used as the metal sheet in the present invention,the JIS 3000 or 5000 series are preferred because of their economy andformability. It is also preferred that the aluminum sheets be subjectedto conventional surface treatments, such as electrolysis, dipping inchromic acid solution, etching in alkaline or acidic solution, or anodicoxidation.

As with the steel sheets, it is also possible to apply a dual-layer,chromium-chromium oxide coating to the surface of the aluminum sheets.In either case, the coating weight of the hydrated chromium oxide ispreferably between 3 and 50 mg/m², (inclusive) chromium, optimallybetween 7 and 25 mg/m² (inclusive). The coating weight of the metallicchromium layer ranges from 10 to 200 mg/m², preferably 30 to 100 mg/m².

Once the metal sheet has been prepared and coated, the polyester resinfilm is applied according to the following procedure:

1. A continuous supply of metal is heated to a temperature exceeding themelting point of the polyester resin;

2. A continuous supply of biaxially oriented polyester resin film iscontacted with the heated metal strip. The resin film may be applied toone or both sides of the metal strip. Optionally, a thermosetting resin(e.g., epoxy resin) can be inserted between the metal sheet and theresin film;

3. The resin film and heated metal strip are then pressed between twolaminating rolls forming a nip at the exit site thereof;

4. The laminating rolls pinch and press the film and the metal strip toensure adhesion;

5. The resin-metal laminate is then cooled at a rate of 600° C./secondas it emerges from the nips of the laminating rolls.

In the above process, the heated metal sheets conduct sufficient heat tomelt the polyester resin film thereto. The resin loses a greater degreeof biaxial orientation nearer its point of contact with the heated metalstrip. Biaxial orientation is retained to a greater degree nearer theuppermost surface of the film, farthest from the heated metal strip andclosest to the cooling action of the nips of the laminating rolls.Retention of the resin film's biaxial orientation subsequent tolamination is maximized by controlling the cooling rate of the laminateimmediately following lamination. The cooling rate is determined by thetemperature of the heated metal strip and the laminating rolls, by thesize of contact area between the resin-metal sheet composite and thelaminating roll, and by the duration of contact between the metal stripsand the laminating rolls. The latter value corresponds with the feedrate of the metal strip and laminating rolls. Generally, greater lossesof biaxial orientation occur when the resin film is subjected toexcessive heat, such as when the metal strip and laminating rolls reachhigh temperatures or when the feed rate of the metal strip is relativelyrapid and the nip length (determined by the diameter of the laminatingroll and the elasticity modulus of the roll) is relatively short.

Optimal retention of desired biaxial orientation, wherein when the resinloses a greater degree of biaxial orientation nearer its point ofcontact with the heated metal strip and retains a greater degree of saidorientation nearer the uppermost surface of the film, farthest from theheated metal strip and closest to the cooling action of the nips of thelaminating rolls, is achieved by controlling the rate at which thelaminate is cooled as it emerges from the nips of the laminating rolls.A cooling rate of 600° C./second appears to result in maximal retentionof the resin film's biaxial orientation. Slower cooling rates do notprevent excessive heating of the resin, resulting in greater loss oforientation. This enhances formability, but reduces impact resistancewhen the laminate is heated subsequent to forming.

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

The following example helps further illustrate the preferred embodimentsof the present invention:

A continuously supplied strip of TFS, having a two layer coating of 110mg/m2 metallic chromium and 14 mg/m2 chromium in the form of chromiumhydroxide and possessing a thickness of 0.18 mm and a temper of DR-10,was heated by heating rolls to the temperatures shown in Table 1. Acontinuous sheet of polyester resin film, from film supplying rolls, wasthen contacted with each side of the heated metal sheet. The films andthe metal sheetwere then laminated, pinched and bonded together betweenlaminating rolls maintained at the temperatures shown in Table 1.Immediately thereafter, as the laminate emerged from the nips of thelaminating rolls, said laminate was quenched at the temperatures shownin Table 1. The cooling rate was adjusted by altering the nip length, asdetermined by the diameters of the laminating rolls.

The biaxially oriented resin films contemplated for use in the aboveexample include the following:

1. PET films possessing a thickness of 25 μm and exhibiting the lowcrystallization temperatures disclosed in Table 1; or

2. Copolyester resins consisting of 88 mol % of ethylene terephthalateand 12 mol % ethylene isophthalate (hereinafter PETI), and possessing athickness of 25 μm and exhibiting the low crystallization temperaturesdisclosed in Table 1; or

3. Blended resins with a ratio of 1 part, by weight, polyethyleneterephthalate resin to 0.6 parts, by weight, polybutylene terephthalateresin (hereinafter PET+PBT), possessing a total thickness of 25 μm (20μm lower layer film, 5 μm upper layer film), and manufactured from ablended copolyester resin composed of a lower layer of 94 mol % ethyleneterephthalate and 6 mol % ethylene isophthalate and polybutyleneterephthalate resin (at a ratio, by weight, of 0.8:1), and an upperlayer of 88 mol % ethylene terephthalate and 12 mol % ethyleneisophthalate (hereinafter PES/PETI), and exhibiting the lowcrystallization temperatures disclosed in Table 1.

Various polyester resin films were isolated by exposing the resin-metallaminate to hydrochloric acid solution. Once the TFS substratum wasdissolved, the resin was divided into 5 mm×60 mm test pieces. These weresubjected to a tensile tester at 75° C. at a cross head distance of 20mm and a stress rate of 200 mm/minute, and the data used to determinethe nominal stress-elongation curves, the true stress-true straincurves, and the true stress values corresponding to a true strain of 1.0at 75° C.

The above-disclosed resin-covered metal sheets were further processedaccording to the procedure described below.

The resin-metal laminates were punched out into circular blanks 160 mmin diameter and formed into drawn cans 100 mm in diameter. These canswere then redrawn into cans having a diameter of 80 mm. The 80 mm canswere then simultaneously drawn and stretch formed and ironed to producecans 66 mm in diameter. The upper edges of the cans were then trimmed,and necks and flanges formed, according to conventional methods.

The following parameters were maintained during the formation of thedrawn cans:

1. The clearance between the drawing portion (corresponding to the upperedge of the can) and the ironing portion was 20 mm;

2. The corner curvature radius in a redrawing die was 1.5 times thethickness of the resin-covered metal sheet;

3. The clearance between the redrawing dies and the punch was equal tothe thickness of the resin-covered metal sheet; and

4. The clearance between the ironing portion of the redrawing dies andthe punch was half the thickness of the resin-covered metal sheet.

Once the resin-covered metal sheets were formed into cans, both impactresistance and the resin's tendency to peel away from the metal sheetwere evaluated as described below and in Table 2. All evaluations weremade upon cans possessing true stresses corresponding to true strains of1.0 at 75° C.

1. Resin's Tendency to Peel

The degree to which the inner and outer resin layers peeled away fromthe metal sheet were observed b the naked eye and evaluated using thefollowing key:

: no peeling-off

: slightly peeled off but no problem for practical use

Δ: heavily peeled off

X : peeled off at the whole upper portion of the can body

2. Impact Resistance of the Resin Film Inside the Body of the Can

To measure the impact resistance of the interior resin, the can wasfilled with water, corked, and dropped from a height of 15 cm. The canwas then opened and the water removed. The can was then packed with a 3%solution of NaCl, and a stainless steel cathode rod inserted therein.Approximately 6.3 volts was then applied between the cathode and the canbody (which served as an anode). Impact resistance was determined fromthe value of the current, in mA, running therethrough.

As indicated in Table 2, resin-covered metal sheets possessing truestresses between 3.0 and 15.0 kg/mm², corresponding to a true strain of1.0 at 75° C., exhibit excellent adhesion and impact resistance.

TABLE 1 Characteristics of polyester resins and laminating conditionsResin film Covering conditions of resin film L.T.C.* Heating Temper-Cooling temper- temperature ature of rate after Sample Resin ature ofmetal laminating lamination Number composition (° C.) sheet (° C.) rolls(° C.) (° C./sec)  1 PET 128 310 150 587  2 PET 128 280 150 637  3 PET128 270 150 650  4 PET 128 280 150 637  5 PET 128 290 150 619  6 PET 128300 150 603  7 PET 128 310 150 587  8 PET 130 290 150 619  9 PET 140 270150 650 10 PET 140 280 150 637 11 PET 140 290 150 619 12 PET 140 300 150603 13 PET 140 310 150 587 14 PET 155 270 150 650 15 PET 155 280 150 63716 PET 155 290 150 619 17 PET 155 300 150 603 18 PET 155 310 150 587 19PET 165 290 150 619 20 PETI 177 245 120 655 21 PET + PBT 140 290 150 61922 PES/PETI 140 300 150 603 *L.T.C. (Low Temperature Crystallization)

TABLE 2 Evaluation result of properties of resin covered metal sheetTrue stress Evaluation of covered corresponding metal sheet to truePeeling- strain of 1.0 off of Impact Sample measured at film (byresistance Number 75° C. (kg/mm²) naked eye) (mA) Item  1 2.1 ⊚ 0.95Comp. Ex.^(#)  2 13.9 ◯ 0.01 Example  3 17.2 X unmeasured Comp. Ex.^(#) 4 14.7 ◯ 0.00 Example  5 10.2 ◯ 0.00 Example  6 3.4 ⊚ 0.09 Example  72.3 ⊚ 0.85 Comp. Ex.^(#)  8 9.3 ⊚ 0.00 Example  9 16.5 Δ 0.00 Comp.Ex.^(#) 10 14.3 ◯ 0.00 Example 11 9.1 ⊚ 0.00 Example 12 3.4 ⊚ 0.08Example 13 2.6 ⊚ 0.53 Comp. Ex.^(#) 14 16.8 Δ 0.00 Comp. Ex.^(#) 15 14.4⊚ 0.00 Example 16 9.4 ⊚ 0.00 Example 17 3.3 ⊚ 0.09 Example 18 2.1 ⊚ 0.49Comp. Ex.^(#) 19 8.9 ⊚ 0.00 Example 20 14.7 ⊚ 0.00 Example 21 11.3 ⊚0.01 Example 22 7.5 ⊚ 0.00 Example Remarks: Comp. Ex.^(#) (ComparativeExample)

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without undue experimentation and withoutdeparting from the generic concept. Therefore, such adaptations andmodifications should and are intended to be comprehended within themeaning and range of equivalents of the disclosed embodiments. It is tobe understood that the phraseology or terminology employed herein is forthe purpose of description and not of limitation. The mans and materialsfor carrying our various disclosed functions may take a variety ofalternative forms without departing from the invention. Thus, theexpressions “means to . . . ” and “means for . . . ” as may be found inthe specification above and/or in the claims below, followed by afunctional statement, are intended to define and cover whateverstructural, physical, chemical, or electrical element or structureswhich may now or in the future exist for carrying out the recitedfunction, whether or not precisely equivalent to the embodiment orembodiments disclosed in the specification above; and it is intendedthat such expressions be given their broadest interpretation.

What is claimed is:
 1. A polyester resin film covered metal sheet for acan produced by ironing after drawing and stretch forming exhibiting atrue stress of 3.0 to 15.0 kg/mm², corresponding to a true strain oa 1.0at 75° C., wherein the polyester film retains its biaxial orientationsubsequent to lamination and wherein the polyester resin has a lowtemperature crystallization of 130 to 165° C. and wherein the lowtemperature crystallization value is obtained from the temperature ofthe exothermic peak produced upon heating a quenched sample of the resinin a differential scanning calorimeter.
 2. A polyester resin filmwherein the mechanical characteristics of said film are measured from atest piece of post-lamination polyester resin film obtained bychemically dissolving the metal sheet layer from a preformed laminateproduced according to claim
 1. 3. The polyester resin covered metalsheet for a can produced by ironing after drawing and stretch formingaccording to claim 1, wherein the polyester resin is polyethyleneterephthalate (PET) resin.
 4. The polyester resin covered metal sheetfor a can produced by ironing after drawing and stretch formingaccording to claim 3, wherein said PET resin has a low temperaturecrystallization of 140 to 155° C. and wherein the low temperaturecrystallization value is obtained from temperature of the exothermicpeak produced upon heating a quenched sample of the resin in adifferential scanning calorimeter.
 5. The polyester resin covered metalsheet for a can produced by ironing after drawing and stretch formingaccording to claim 1, wherein the polyester resin is selected from thegroup consisting of: copolyester resins comprised of recurring ethyleneterephthalate units; copolyester resins comprised of recurring butyleneterephthalate units; copolyester resins comprised of at least two ofthese resins; and double-layered polyester resins comprised of alaminate of at least two of these resins.
 6. A method of producing apolyester resin covered metal sheet for a can produced by ironing afterdrawing and stretch forming according to claim 1, wherein the metalsheet is heated to a temperature exceeding the melting point of theresin, the resin is contacted with the metal sheet, the resin and metalsheet are pinched and pressed into a laminate by a pair of laminatingrolls, and the resulting resin-covered metal sheet is immediately cooledat a cooling rate of at least 600° C./second by a nip formed by thelaminating rolls.
 7. The polyester resin film covered metal sheet for acan produced by ironing after drawing and stretch forming according toclaim 6, wherein the polyester resin is polyethylene terephthalate (PET)resin.
 8. The polyester resin covered metal sheet for a can produced byironing after drawing and stretch forming according to claim 7, whereinsaid PET resin has a low temperature crystallization of 140 to 155° C.and wherein the low temperature crystallization value is obtained fromtemperature of the exothermic peak produced upon heating a quenchedsample of the resin in a differential scanning calorimeter.
 9. Thepolyester resin covered metal sheet according to claim 6, wherein thepolyester resin is selected from the group consisting of: copolyesterresins comprised of recurring ethylene terephthalate units; copolyesterresins comprised of recurring butylene terephthalate units; copolyesterresins comprised of at least two of these resins; and double-layeredpolyester resins comprised of a laminate of at least two of theseresins.